I've used it for quite some time. But it's having this issue.
When I'm using the clean channel, all things seem alright, but when I use any other setting like twang, crunch, metal, insane etc, after maybe 20-40 seconds, the effects stop working, and it goes straight back to clean (while the LED still shows that the effect is still active) Also the volume drops significantly.

If it's an MKI the first thing to try is parking ALL of the knobs to "ZERO" after you have everything set (except for the MASTER VOL and PRESENCE knobs). The reason is to keep the knobs from vibrating loose from their set position. One little nudge is all it takes to get the amp going crazy on you! Or, you can try placing the head somewhere else besides on top of the cab (I'm assuming you have it there) or where the loud sounds wont affect it. I have my Marshall 2x12 cab in a case and I place my MKI on top of that for live shows. Doubles as a pick, slide and beer shelf, too!